The present invention relates generally to the field of cardiac rhythm management and more particularly to a dual chamber cardiac pacemaker incorporating a system for discriminating between pacemaker mediated tachycardia (PMT) and simple upper rate limit Wenckebach behavior due to normal sinus tachycardia. The system may also provide for adjusting the upper rate limit (URL) or maximum tracking rate (MTR) according to the frequency of detected pacemaker Wenckebach events.
In dual chamber pacemakers, circuitry is provided for both sensing atrial and ventricular depolarization events and for pacing one or both of atrial and ventricular tissue. In a subject with normal sinus node activity and interrupted conduction system, the pacemaker is able to sense an atrial depolarization (P-wave) and thereafter stimulate the ventricle in accordance with an established AV delay interval. This effectively mimics the heart""s PR interval. The situation is complicated, however, by the possible occasional occurrence of an interfering retrograde conducted P-wave, possibly the result of a ventricular stimulating pulse, but one which is also sensed by the atrial sensing circuitry. Because the atrial sensing circuitry of the pacemaker cannot tell whether a sensed signal is a normal or retrograde conducted P-wave it will initiate another ventricular stimulation event. This may quickly lead to PMT. To overcome this problem, dual chamber pacemakers are typically programmed to include a post-ventricular atrial refractory period (PVARP) during which atrial events are sensed but ignored. In this manner, if an atrial event occurs during PVARP due to retrograde conduction, an AV interval is not initiated and no ventricular stimulating pulse is generated as a result of the atrial event.
The addition of PVARP does not totally successfully resolve the problem either, however, because in many pacemaker treated patients the condition of the patient is such that the retrograde conduction time varies or fluctuates depending upon physiologic feedback mechanisms. This means that a fixed, programmable PVARP may become relatively too short over time if retrograde conduction time increases and may no longer serve to inhibit PMT. Conversely, if the PVARP is programmed to be too long, this shortens the sensing window and as the pacing rate reaches the maximum atrial tracking rate or MTR set for the pacemaker, some of the desirable P-waves will fall inside the PVARP and be ignored and this will result in an undesirable drop in the ventricular pacing rate. This is known as a two-to-one block. Thus, each time a P-wave falls within PVARP and an AV block occurs for that cardiac cycle it results in a missing cardiac cycle which is undesirable because it causes short-term loss of AV synchrony and the subsequent loss of cardiac output.
Pacemaker Wenckebach is another type of undesirable upper rate limit behavior which negatively affects the patient. In pacemaker Wenckebach, as the atrial rate increases, the AV interval is lengthened so that the ventricular pacing interval does not exceed the MTR. As the atrial rate increases, a P-wave will eventually fall within PVARP and AV block will occur for that cardiac cycle. The successive lengthening of the AV interval leading to a missing cardiac cycle likewise causes short-term loss of AV synchrony and subsequent loss of cardiac output. The detection of pacemaker Wenckebach is important as an indication of a possible need for URL/MTR adjustment.
Pacemakers are implanted to typically operate within a particular beat rate or heart rate (HR) range including a particular URL or MTR which is typically picked on the conservative side by the physician who may have a minimum familiarity with the level of activity reached by the patient. The actual proper HR range for the patient may well extend above that initially programmed. This being the case, the pacer may repeatedly reach the URL or MTR because of naturally occurring sinus rhythms and pacemaker Wenckebach may then occur. (As used herein, the term Wenckebach, unless otherwise stated, refers to a pacemaker Wenckebach epidsode). Because URL or MTR may also be reached due to PMT, and PMT should be stopped as soon as possible, because of the alternate loss of cardiac output, there is a definite need to distinguish between these two phenomena.
Pacemakers have been programmed with a function to determine whether a patient is in a pacemaker mediated tachycardia (PMT) by counting intervals that are atrially sensed and ventricularly paced at the URL. After a prescribed number of successive intervals are perceived to be at the maximum rate, perhaps 16, the pacemaker assumes the existence of PMT and is programmed to extend the next PVARP a sufficient time to break or interrupt the PMT. This PVARP extension is typically about 400-500 ms. An example of this approach is found in Walmsley et al (U.S. Pat. No. 5,674,255) assigned to the same Assignee as the present invention. The contents of that patent are deemed incorporated by reference herein for any purpose. While this solution has been successful in interupting and correcting actual PMT, it is unable to predictably discriminate between URL events which are caused by PMT and which occur simply because the patient exercised to the MTR/URL.
Thus, while the above determination and aleviation of PMT has been quite successful, a need remains for adding a technique which would produce a better and more sophisticated analysis of the nature of the cause of the upper rate limit behavior. In this manner, if one could reliably determine whether the rhythm is a PMT or a Wenckebach episode due to normal sinus tachycardia, after a given number of Wenckebach events within a prescribed time, the URL or MTR could be adjusted upward so that the pacemaker range would self-adjust to be more in line with the actual activity level of the patient.
Further, in describing the related art and features of the present invention, it is believed that it would be helpful to define certain terminology. Accordingly, several definitions are presented.
Maximum Tracking Rate (MTR) or Upper Rate Limit (URL) is the maximum rate at which the paced ventricular rate will track sensed atrial events. It is applicable to the atrial synchronous pacing modes, DDD, DDDR, VVDR and VDD and is programmable quantity typically residing in the range of from about 50 to 185 pulses per minute.
AV Delay (AV) is the programmable time period from the occurrence of an atrial event, either sensed or paced, to a paced ventricular event. It is a programmable quantity typically ranging between 10 and 300 milliseconds and is active in DDD, DDI, DVI, DOO, VDD and the similar rate responsive modes.
Post Ventricular Atrial Refractory Period (PVARP) is defined as the time period after a ventricular event, either paced or sensed, during which activity in the atrium does not inhibit an atrial stimulation pulse nor trigger a ventricular stimulating pulse. It is designed to avoid atrial sensing of retrograde activity initiated in the ventricle.
VA Interval is defined as the time period from the occurrence of a ventricular event, either paced or sensed, to the occurrence of an atrial event, either sensed or paced.
Pacemaker Mediated Tachycardia (PMT). In DDD(R) and VDD(R) pacing modes, the pacemaker may detect retrograde conduction in the atrium, causing triggered ventricular pacing rates as high as the MTR. This is referred to in the literature as pacemaker-mediated tachycardia or endless loop tachycardia.
Total Atrial Refractory Period (TARP) is defined as the sum of the AV delay and PVARP.
The present invention includes a system and method for dealing with periodic patterns of tachycardia by more accurately determining the origin of the upper rate limit (URL) behavior. The system discriminates between PMT and normal sinus tachycardia and includes the capacity to adjust the URL/MTR upward by increments, possibly one, five or ten or even up to thirty beats per minute in response to a predetermined frequency of Wenckebach events, based on as few as possibly three or more events per month up to ten or more events per week.
If events are determined to be PMT, the system also may act to break episodes of PMT by increasing the PVARP interval to a predetermined longer time, say 400-500 ms, a value that insures that retrograde P-waves are not tracked for at least one beat.
As an alternative in accordance with the present invention, the PVARP may be extended by a value equal to the measured retrograde conduction time plus some constant time, particularly between about 5 ms and 100 ms, such as 50 ms. This method minimizes the PVARP extension necessary to terminate the PMT.
Pacemakers of the class in which the invention is generally applicable include a pacer control algorithm designed to determine whether a tachycardia event is a PMT or a Wenckebach event. According to the invention, a VA stability check is added to a monitoring system that counts a predetermined number of intervals, generally 16, that have been atrially sensed and ventricularly paced at the preset URL or MTR of the pacemaker and which are normally utilized to determine PMT. In accordance with the VA stability check of the invention, it has been determined that if the VA interval varies by more than a minimum amount, typically in the range of 5-50 ms, during a series of monitored beats at URL or MTR, then the event is more likely to have been caused by a Wenckebach function than by PMT. This particularly may be the case if successive shortenings of the interval are noticed during the onset of tachycardia. The retrograde conduction time generally lengthens as the beat rate increases with increased exercise and shortens with decreased activity. It has been found that if the VA interval varies by more than about 10-15%, or about 30 ms from beat to beat; the rate change is almost always found to be charcteristic of a Wenckebach event. However, even change in the range from about 5 ms to 50 ms may well be due to a natural increase in the patient""s intrinsic atrial rate and changes above about 20 ms have a higher probability of being due to pacemaker Wenckebach. Thus, a deviation range from about 5 ms to 50 ms has a reasonably chance of being due to natural increases in the patient""s intrinsic atrial rate; while changes from about 20 ms to 50 ms have a high probability and at 30 ms or more, the deviation is almost certain to be due to natural increases in atrial rate. This value has been selected as a non-limiting example in the detailed description below.
One successful detection system uses 32 ms as the threshold beat-to-beat VA change. This value represents about 10-15% of a typical VA interval.
If a number of naturally induced or Wenckebach events occur within a given time period, say 10 per week, or even 3 per month, this may well be indicative that a condition exists in which the URL/MTR is set too low and should be incremented by one, five or 10 or more beats per minute to better represent the actual required pacing rate range of the heart of the patient. The control program of the pacer according to the present invention may also contain the ability to adjust the URL/MTR upward from one to about 30 RPM automatically in response to the situation.
Accordingly, it is a primary object of the present invention to provide a technique that more accurately distinguishes between upper rate limit behavior precipitated by a PMT and a pacemaker Wenckebach event.
It is a further object of the present invention to provide a pacemaker Wenckebach counter which indicates a cumulative number of Wenckebach events in a given time which may be used to indicate that the pacemaker is programmed at too low a URL or MTR.
A still further object of the present invention is to utilize such a Wenckebach counter as an input to a system that operates to automatically alter the pacing limit to increase the URL/MTR by a certain number of beats per minute.
These and other objects, as well as these and other features and advantages of the present invention, will become readily apparent to those skilled in the art from a review of the following detailed description of the illustrated embodiment in conjunction with the accompanying drawings in which like numerals in the several views refer to corresponding parts.